This invention relates generally to fuel cells, and more particularly to a method of maintaining a neutral water balance in a fuel cell for an automobile.
In automotive systems incorporating fuel cells, it is important to maintain neutral water balance so that the vehicle does not run out of process water in the event of net water consumption. Generally, automotive fuel cell systems require a condenser at the exhaust in order to recover process water and maintain a neutral water balance in the cell. Condensers are relatively large and occupy a substantial volume in such systems. Accordingly, it would be desirable to provide a method for operating a fuel cell in which neutral water balance can be maintained without the need for a condenser.
It is therefore an object of the invention to provide a fuel cell assembly that minimizes breakdown and damage due to fuel cell water depletion.
It is a further object of the invention to maintain sufficient water content while operating the fuel cell in an efficient manner.
It is an even further object of the invention to optimize performance while maintaining sufficient water content.
It is a still further object of the invention to reduce overall weight and space requirements for a fuel cell while maintaining sufficient water content in the fuel cell.
It is still another object of the invention to simplify the support system for a fuel cell while maintaining sufficient water content in the fuel cell.
These and other objects of the invention are achieved by a fuel cell assembly that includes a water balance control system that effects water production or consumption by a fuel cell necessary to return to a balanced state within the fuel cell. The water balance control system causes such production or consumption through adjustment of the fuel cell operating temperature. The fuel cell operating temperature can be adjusted by controlling the degree of heat transfer from the fuel cell coolant. Preferably, this heat transfer is controlled through a fan cooled radiator in the coolant loop.
The water balance control system can make the necessary temperature adjustments in response to one or more of a variety of factors that are indicative of the water balance state of the fuel cell. In one embodiment of the invention, a fuel cell system is operated at a reduced temperature, for example approximately 58.5xc2x0 C. at 30 psig under ambient conditions of 30 degrees C., 60% R.H. and 1 atm pressure, enabling the system to recover more water directly from the fuel cell and thereby eliminate the need for a condenser to recover water from the exhaust gases in the automotive system.
In a further embodiment of the invention, the fuel cell operating temperature is varied in response to measured ambient conditions (such as temperature, pressure and relative humidity) with closed loop water balance in response to load with respect to operating conditions, such as load, stack temperature and stack pressure. For example, the fuel cell operating condition can be adjusted to maintain water level in a reservoir within a predetermined range. The invention can also incorporate a combination of the approaches.
A main advantage is the elimination of a condenser and any associated fan, with a corresponding reduction in system size, complexity and parasitic load. Another advantage of lower fuel cell operating temperatures is that the amount of water needed to humidify the cathode air is reduced. Thus, the heat generated from compressed air and the reformer excess water (or downlet) can be sufficient to vaporize the water and directly humidify the cathode air.